Estimation Of Synchronization Signal Using Sinusoidal Amplitude Integrators In Synchronous Reference Frame

Energy crisis and environmental pollution push the researchers to make effortstowards the development of distributed generation (DG) systems, such as wind andsolar energy to address energy and environmental problems. The morden grid requiresthat DG systems should be grid-connected even under non-ideal grid conditions.Therefore, as the interface of the DG systems, the control of the grid-connectedconverters is particularly important; actually, the fast and accurate detection of gridsynchronization signals is a key issue that has to be considered in the control of theconverter and a lot of synchronization methods have been proposed, among theseexisted methods, SRF-PLL is the most widely used and it outputs accuratesynchronization signal under ideal grid condition, however, SRF-PLL can not workwell when the grid voltage is contaminated by unbalance, harmonics and dc offsets,etc.. To realize an accurate and fast extraction of grid synchronization signals, based onthe SRF-PLL and complex-coefficient transfer function (CCF), a brandly new PLLbased synchronization method is proposed in this paper and its operation principle isfully analyzed.Under unbalanced grid condition, in synchronous reference frame, thepositive-sequence component corresponds to dc, while the negative-sequencecomponent corresponds to the ac at twice the fundamental frequency of opposite sign(-2ω0), which deteriorates the output of the SRF-PLL. To obtain an accurate extractionof synchronization signals, some measures should be taken to accurately extract the dccomponent. The sinusoidal amplitude integrator (SAI) is a resonant integrator who canrealize the amplitude integration of sinusoidal signals; what’s more, the SAI possessesthe frequency and the polarity selective property, so, simillar to the dc case, the SAIbased feedback loop can extract the specified ac component with unity gain and zerophase shift. Therefore, the SAI-Based PLL method is proposed by incorporating theSAI based feedback loop and its relevant auxiliary separation unit into the SRF-PLLstructure. With the proposed method, the dc and ac component can be separatedcompletely in single synchronous reference frame, therefore, the amplitude and phaseangle of both positive-and negative-sequence components can be extracted. In thispaper, the operation of the proposed method is analyzed using the complex-vectornotation. Simulation and experimental results on the performance of the proposed SAI-Based PLL and comparison with other synchronization methods are presented.In d-q frame, the dc offset generates a voltage component having the samefrequency (-ω0) as that of the grid voltage. By changing the resonant frequency of theSAI, the proposed SAI-Based PLL can also completely eliminate the influence of thedc offset contained in the input grid voltage. Moreover, this paper presents a generalMulti-SAI-Based PLL structure for grid synchronization when the grid is unbalanced,harmonically distorted, and contains dc offset. Analysis is given to demonatrate thatthe Multi-SAI-Based PLL can eliminate the influence of unbalance, harmonics, and dcoffset at the same time. Simulation and experimental results are presented todemonstrate the excellent performance of the proposed method.